I had the same issue (masse light) with my Marco UP3E pump. I would be very interested in what you may have learned..
I also removed the bonding wire from the pump to solve the problem. However, a year later the pump stopped running in a place I could not easily replace it. I disassembled the motor and found the motor brush wire was completely corroded and had fallen apart. I'm guessing that the corroded wire was touching the case??? As a temporary repair I soldered in a new brush wire, but I don't expect it to work for long.
I'm not sure what caused the brush wire to corrode. Only one of them did, the other was fine. Now I need to replace the pump and I am not sure I want another Marco pump. The cause of the corrosion could be external to the pump I suppose. There were not any leaks wetting the pump. The only thing I can think of causing the corrosion was the plumbing fittings I used. They were advertised as 316 stainless steel, but they are corroding internally like mad. I'm replacing that plumbing with Pex, plastic fittings and stainless pinch rings.